Chair base



FIG. 1 is a perspective view showing the top, front and left side of a chair base which forms my new design;

FIG. 2 is an enlarged top plan view thereof;

FIG. 3 is an enlarged front elevational view thereof;

FIG. 4 is an enlarged rear elevational view thereof;

FIG. 5 is an enlarged right side elevational view thereof;

FIG. 6 is an enlarged left side elevational view thereof; and,

FIG. 7 is an enlarged perspective view thereof showing the top, front and left side thereof.

The broken lines show portions of a chair that form no part of the claimed design. 

The ornamental design for a chair base, as shown and described. 